New Carseat?

My DD is very tall for her age she is 28 3/4 inches and 18lbs *she had her 6 month well visit last week* According to the limits for her infant car seat it is time for us to invest in a new one (I can't believe how fast that went!!!) Have any of you bought the next size? Did you get the covertible kind? What brand would you recommend?

We also love the britax marathon. We used it with our two older kids and are shopping for one for our May baby.
You have to buy a convertible one, since they have to be rear facing until they are one, or even better until they're two.